Drop support for Python 2.7

[Technologicat]

Python 2.7 will be out of support January 1, 2020.

Dropping support for it allows a simple solution for #1.

A possibly incomplete list of things to remember:

• Update metadata in setup.py.
• Update GitHub tags.
• Update hashbangs.
• Drop runtests2.sh.
• Consider migrating runtests3.sh into a runtests.py. Shell? Where we're going, we need no shell.
  • Unpythonic has a template for this.
  • Or could just use unittest from the stdlib.
  • pytest would be even nicer, but introduces a dependency.